A grain-oriented electrical steel sheet contains Si, and axes of easy magnetization (cubic crystal ((100)<001>) of crystal grains in the steel sheet are substantially parallel to a rolling direction in a manufacturing process of the steel sheet. The grain-oriented electrical steel sheet is excellent as a material of iron core of a transformer and the like. Particularly important properties among magnetic properties of the grain-oriented electrical steel sheet are a magnetic flux density and an iron loss.
There is a tendency that a magnetic flux density of the grain-oriented electrical steel sheet when a predetermined magnetizing force is applied is larger, as the degree in which the axes of easy magnetization of crystal grain are parallel to the rolling direction (which is also referred to as L direction) of the steel sheet is higher, namely, as the matching degree of crystal orientation is higher. As an index for representing the magnetic flux density, a magnetic flux density B8 is generally used. The magnetic flux density B8 is a magnetic flux density generated in the grain-oriented electrical steel sheet when a magnetizing force of 800 A/m is applied in the L direction. Specifically, it can be said that the grain-oriented electrical steel sheet with a large value of the magnetic flux density B8 is more suitable for a transformer having small size and excellent efficiency, since it has a large magnetic flux density generated by a certain magnetizing force.
Further, as an index for representing the iron loss, an iron loss W17/50 is generally used. The iron loss W17/50 is an iron loss obtained when the grain-oriented electrical steel sheet is subjected to AC excitation under conditions where the maximum magnetic flux density is 1.7 T, and a frequency is 50 Hz. It can be said that the grain-oriented electrical steel sheet with a small value of the iron loss W17/50 is more suitable for a transformer, since it has a small energy loss. Further, there is a tendency that the larger the value of the magnetic flux density B8, the smaller the value of the iron loss W17/50. Therefore, it is effective to improve the orientation of crystal grains also for reducing the iron loss W17/50.
Generally, the grain-oriented electrical steel sheet is manufactured in the following manner. A material of silicon steel sheet containing a predetermined amount of Si is subjected to hot-rolling, annealing, and cold-rolling, so as to obtain a silicon steel sheet with a desired thickness. Then, the cold-rolled silicon steel sheet is annealed. Through this annealing, a primary recrystallization occurs, resulting in that crystal grains in a so-called Goss orientation in which axes of easy magnetization are parallel to the rolling direction (Goss-oriented grains, crystal grain size: 20 μm to 30 μm) are formed. This annealing is performed also as a decarburization annealing. Thereafter, an annealing separating agent containing MgO as its major constituent is coated on a surface of the silicon steel sheet after the occurrence of primary recrystallization. Subsequently, the silicon steel sheet coated with the annealing separating agent is coiled to produce a steel sheet coil, and the steel sheet coil is subjected to an annealing through batch processing. Through this annealing, a secondary recrystallization occurs, and a glass film is formed on the surface of the silicon steel sheet. When the secondary recrystallization occurs, due to an influence of inhibitor included in the silicon steel sheet, the crystal grains in the Goss orientation preferentially grow, and a large crystal grain has a crystal grain size of 100 mm or more. Then, an annealing is performed for flattening the silicon steel sheet after the occurrence of secondary recrystallization, a formation of insulating film and the like, while uncoiling the steel sheet coil.
Almost all of the orientations of respective crystal grains of the grain-oriented electrical steel sheet manufactured through such a method are determined when the secondary recrystallization occurs. FIG. 1A is a diagram illustrating orientations of crystal grains obtained through the secondary recrystallization. As described above, when the secondary recrystallization occurs, crystal grains 14 in the Goss orientation, in which a direction 12 of the axis of easy magnetization matches a rolling direction 13, preferentially grow. At this time, if the silicon steel sheet is not flat and is coiled, a tangential direction of a periphery of the steel sheet coil matches the rolling direction 13. Meanwhile, the crystal grains 14 do not grow in accordance with curvature of the coiled steel sheet surface but grow while maintaining a linearity of the crystal orientation in the crystal grains 14, as illustrated in FIG. 1A. For this reason, when the steel sheet coil is uncoiled and flattened after the occurrence of secondary recrystallization, a part in which the direction 12 of the axis of easy magnetization is not parallel to the surface of the grain-oriented electrical steel sheet is generated in a large number of crystal grains 14. In short, an angle deviation β between the axis of easy magnetization direction (cubic crystal (100)<001>) of each crystal grain 14 and the rolling direction is increased. When the angle deviation β is increased, the matching degree of crystal orientation is decreased, and the magnetic flux density B8 is decreased.
Further, the larger the crystal grain size, the more significant the increase in the angle deviation β. In recent years, because of strengthening of inhibitors and the like, it is possible to facilitate a selective growth of crystal grains in the Goss orientation, and in a crystal grain having a large size in the rolling direction in particular, the decrease in the magnetic flux density B8 is significant.
Further, various techniques have been conventionally proposed for the purpose of improving the magnetic flux density, reducing the iron loss or the like. However, with the conventional techniques, it is difficult to achieve the improvement in the magnetic flux density and the reduction in the iron loss, while maintaining high productivity.